1. Field of the Invention
The present invention relates to an electromagnetic within-mold stirring method designed to improve the quality of ingots obtained by horizontal continuous casting, and more particularly relates to an electromagnetic within-mold stirring method and an apparatus therefor, designed to minimize occurrence of surface defects such as cold shut and vertical surface cracks.
2. Description of the Prior Art
Intensive studies have been made in various countries for development and practical use of horizontal continuous casting, and investigation has also been made of applying electromagnetic stirring thereto for the same purpose as in secondary cooling zone stirring in vertical continuous casting such as an ordinary upright bending type casting and a curved type continuous casting, that is, for the purpose of increasing the equi-axed crystal zone and remedying central segregation.
For example, Japanese Patent Application Disclosure Nos. 120453/1977, 89829/1978 and 1544/1982 propose methods of stirring molten steel within a mold in horizontal continuous casting.
However, very few can be put to practical use, and electromagnetic stirring has not yet been developed to the extent that its effect can be fully enjoyed. Horizontal continuous casting machines are inevitably operated by intermittent drawing because of their construction being entirely different from that of vertical continuous casting machines, but intermittent drawing entails a surface defect called cold shut, said defect remaining even after rolling. This accounts for the fact that scarfing or cutting of the ingot surface has been practiced with the full knowledge of an inevitable decrease in the yield of good ingots; thus it has been desired to establish measures to prevent occurrence of cold shut itself. Accordingly, a method has been proposed which uses a break ring which causes the inner diameter of the refractory between the mold and the nozzle to approach the inner diameter of the mold. With this method, however, there is a problem in that drawing becomes impossible as the break ring is consumed and thus such is not suitable for a long-term operation. There has also been proposed to increase the drawing cycle so as to decrease the cold shut depth, but the effect of decreasing the cold shut depth is insufficient and there is a disadvantage in that the drawing mechanism becomes too complicated. On the other hand, in the case of horizontal casting of round billets, cooling of the upper and lower surfaces of the billet in the mold tends to be nonuniform, resulting in longitudinal surface cracks in the upper surface, which is insufficiently cooled. However, there has been no report regarding measures to prevent this drawback.